Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
In an electric current producing cell or battery, discharge of the cell from its charged state occurs by allowing electrons to flow from the anode to the cathode through an external circuit resulting in the electrochemical reduction of the cathode active material at the cathode and the electrochemical oxidation of the anode active material at the anode. Under undesirable conditions, electrons may flow internally from the anode to the cathode, as would occur in a short circuit. To prevent this undesirable internal flow of electrons that occurs in a short circuit, an electrolyte element is interposed between the cathode and the anode. This electrolyte element must be electronically non-conductive to prevent the short circuiting, but must permit the transport of positive ions between the anode and the cathode during cell discharge, and in the case of a rechargeable cell, during recharge. The electrolyte element should also be stable electrochemically and chemically towards both the anode and the cathode.
Typically, the electrolyte element contains a porous material, referred to as a separator since it separates or insulates the anode and the cathode from each other, and an aqueous or non-aqueous electrolyte in the pores of the separator. The aqueous or non-aqueous electrolyte typically comprises ionic electrolyte salts and electrolyte solvents, and optionally, other materials such as for example, polymers. A variety of materials have been used for the porous layer or separator of the electrolyte element in electric current producing cells. These porous separator materials include polyolefins such as polyethylenes and polypropylenes, glass fiber filter papers, and ceramic materials. Usually these separator materials are supplied as porous free standing films which are interleaved with the anodes and the cathodes in the fabrication of electric current producing cells. Alternatively, the porous separator layer can be applied directly to one of the electrodes, for example, as described in U.S. Pat. No. 5,194,341 to Bagley et al.
Porous separator materials have been fabricated by a variety of processes including, for example, stretching combined with special heating and cooling of plastic films, extraction of a soluble plasticizer or filler from plastic films, and plasma oxidation. The methods for making conventional free standing separators typically involve extrusion of melted polymeric materials either followed by a post-heating and stretching or drawing process or followed by a solvent extraction process to provide the porosity throughout the separator layer. U.S. Pat. No. 5,326,391 to Anderson et al. and references therein, describe the fabrication of free standing porous materials based on extraction of a soluble plasticizer from pigmented plastic films. U.S. Pat. No. 5,418,091 to Gozdz et al. and references therein, describe forming electrolyte layers by extracting a soluble plasticizer from a fluorinated polymer matrix either as a coated component of a multilayer battery structure or as an individual separator film. U.S. Pat. No. 5,194,341 to Bagley et al. describes an electrolyte element with a microporous silica layer and an organic electrolyte. The silica layer was the product of the plasma oxidation of a siloxane polymer.
These manufacturing methods for free standing separators are complex and expensive and are not effective either in providing ultrafine pores of less than 1 micron in diameter or in providing separator thicknesses of less than 15 microns. The methods for making a separator coated directly on another layer of the cell typically involve a solvent extraction process after coating to provide the porosity throughout the separator layer. As with the free standing separators, this solvent extraction process is complex, expensive, and not effective in providing ultrafine pores of less than 1 micron in diameter.
Carlson et al. in U.S. patent application Ser. No. 08/995,089 to the common assignee, describe separators for use in electrochemical cells which comprise a microporous pseudo-boehmite layer and electrolyte elements comprising such separators. The pseudo-boehmite separators and methods of making such separators are described for both free standing separators and as a separator layer coated on an electrode.
As the non-aqueous electrolyte in the pores of the separator of an electrolyte element, a liquid organic electrolyte comprising organic solvents and ionic salts is typically used. Alternatively, a gel or solid polymer electrolyte containing polymers and ionic salts, and optionally organic solvents, might be utilized instead of the liquid organic electrolyte. For example, U.S. Pat. Nos. 5,597,659 and 5,691,005 to Morigaki et al. describe a separator matrix formed of a microporous polyolefin membrane impregnated in its pores with an ionic conductive gel electrolyte.
In addition to being porous and being chemically stable to the other materials of the electric current producing cell, the separator should be flexible, thin, economical in cost, and have good mechanical strength. These properties are particularly important when the cell is spirally wound or is folded to increase the surface area of the electrodes and thereby improve the capacity and high rate capability of the cell. Typically, free standing separators have been 25 microns (.mu.m) or greater in thickness. As batteries have continued to evolve to higher volumetric capacities and smaller lightweight structures, there is a need for separators that are 15 microns or less in thickness with a substantial increase in the area of the separator in each battery. Reducing the thickness from 25 microns to 15 microns or less greatly increases the challenge of providing porosity and good mechanical strength while not sacrificing the protection against short circuits or not significantly increasing the total cost of the separator in each battery.
This protection against short circuits is particularly critical in the case of secondary or rechargeable batteries with lithium as the anode active material. During the charging process of the battery, dendrites can form on the surface of the lithium anode and can grow with continued charging. A key feature of the separator in the electrolyte element of lithium rechargeable batteries is that it have a small pore structure, such as 10 microns or less in pore diameter, and sufficient mechanical strength to prevent the lithium dendrites from contacting the cathode and causing a short circuit with perhaps a large increase in the temperature of the battery leading to an unsafe explosive condition.
Another highly desirable feature of the separator in the electrolyte element is that it is readily wetted by the electrolyte which provides the ionic conductivity. When the separator material is a polyolefinic material, which has nonpolar surface properties, the electrolytes (which typically have highly polar properties) often poorly wet the separator material. This results in low capacities in the battery due to the nonuniform distribution of the electrolyte in the electrolyte element.
Further it would be highly advantageous to be able to prepare free standing separators by a relatively simple process of coating which directly provides ultrafine pores as small as 1 nm in diameter and can readily provide a range of thicknesses from 50 microns or greater down to 1 micron. Also, it would be advantageous to be able to prepare separators with ultrafine pores and a wide range of thicknesses coated directly on another layer of the electric current producing cell by a process of coating without requiring any subsequent solvent extraction or other complex process which is costly, difficult to control, and not effective in providing ultrafine pores.
A separator, particularly one with a thickness less than 25 microns, which is applicable for electric current producing cells, and which can avoid the foregoing problems often encountered with the use of polyolefinic and other conventional porous materials made using extrusion, extraction, or other processes would be of great value to the battery industry.